Starter motor assemblies that assist in starting engines, such as engines in vehicles, are well known. The conventional starter motor assembly broadly includes an electrical motor and a drive mechanism. The electrical motor is energized by a battery upon the closing of an ignition switch. The drive mechanism transmits the torque of the electric motor through various components to a flywheel gear of the engine, thereby cranking the engine until the engine starts.
During operation of the starter motor, the closing of the ignition switch (typically by turning a key) energizes a solenoid coil and, in some motors, applies some power to the electrical motor. Energization of the solenoid coil moves a solenoid core member (also referred to herein as the “plunger”) in an axial direction. The movement of the solenoid plunger closes electrical contacts, thereby delivering full power to the electrical motor. The movement of the solenoid plunger also biases a pinion-type gear into engagement with the engine flywheel gear. Engagement of the rotating pinion with the flywheel in turn causes the flywheel to rotate, thereby cranking the vehicle engine. Once the vehicle engine is started, the operator of the vehicle then will open the ignition switch, which deenergizes the solenoid assembly. As a result of this deenergization, the magnetic field that caused the plunger to move decreases and at some point is overcome by a return spring.
In order for energization of the solenoid assembly to both move the solenoid plunger toward the flywheel and hold the plunger in place during pinion-flywheel engagement, solenoid assemblies often utilize two coils, i.e., a pull-in coil and a hold-in coil. In these arrangements, both coils are energized in order to bias the plunger in the axial direction for engagement with the engine flywheel. Once the plunger engages the electrical contacts such that full power is delivered to the starter motor, the pull-in coil is effectively short circuited, eliminating unwanted heat generated by the coil. The hold-in coil then holds the plunger in place in order to hold the pinion in the engagement position with the flywheel until the engine starts.
In designing solenoid coil windings for a starter motor, including the design of pull-in coils and the hold-in coils, design challenges are encountered that relate to the physical dimensions of the coils, the electrical resistance of the windings, and the resulting amp-turn excitation that each coil provides. For example, in the case of a pull-in coil, it may be desirable to increase resistance in the coil without increasing the resulting amp-turn excitation of the coil. Increasing the resistance of the pull-in coil without increasing the resulting amp-turn excitation allows the coil to have a desired resistance and still provide a desired amount of amp-turn excitation for proper movement of the plunger within the solenoid. This allows the amp-turns for the pull-in coil and the hold in coil to be properly balanced such that a spring force on the plunger will return the plunger to its original position at engine start.
In meeting the foregoing design challenge of increased resistance without increased amp-turn excitation, reverse turns are often used in solenoid arrangements. FIGS. 6A-6C show an exemplary conventional coil and spool capable of providing a reverse turn in the coil windings. In these arrangements, a conductor 132 is first wound around a spool 150 to provide a first layer 134 of windings for a coil. Next, the conductor 150 is wrapped around a hook 158 on the flange of the spool. This allows the next layer 136 of the coil (shown incomplete in FIGS. 6B and 6C) to be wound in the opposite direction from the first layer 134. These two layers 134, 136 of coil wound in opposing directions result in a net amp-turn excitation of effectively zero, as the opposing excitation provided by the first two layers of windings cancel each other out.
While the above-described arrangement for increasing resistance without increasing the amp-turn excitation of the coil provides some level of flexibility for the designer, sometimes this level of flexibility isn't sufficient. For example, design constraints may not allow two full layers of reversing turns either from a spatial or resistance standpoint. Furthermore, conventional designs do not facilitate a design where reversing turns may be provided on the innermost layer of the coil. Accordingly, it would be desirable to provide a solenoid arrangement for a starter that allows the designer additional flexibility in providing the optimal resistance and amp-turn excitation of a coil. It would also be desirable if such a solenoid arrangement were relatively simple and inexpensive to implement.